After several instances of popping its fuse (and tripping the mains circuit breaker) I pulled it out of the rack and have been waiting for a chance to look at it. Opened it up today, popped in a higher value fuse than previously (500mA rather than 200mA) and it didn’t pop anything, but it didn’t power up either.

I’m not inclined to troubleshoot the power supply because a) the PSU (not least the transformers) is well known as the weak spot of these synths and b) someone has modified it in the past and c) it would be pretty awkward to get at all the parts/contacts whilst keeping the unit in a state suitable for sound testing.

Based on this https://www.2btech.co.uk/refimages/ms6_power_184.pdf (obtained from somewhere on the inter-webs in the past) the MS6 requires +/- 5v and +/- 12v. Ignore the 12v AC overlay, that is not my intention. There are no official schematics, so everything for a Cheetah found on the web has to be treated with some caution. But in principle this corresponds with the 7805/7905 7812/7912 regulators in the MS6.

Based on this, and fact that I saw an example of someone doing a similar thing on the web (somewhere), I am wondering if there is a single supply available, ideally in a laptop PSU type format, with a 5 pin connector delivering 0v +/- 5v and +/- 12v. If so, it would probably be best to desolder all of the PSU components and run the external PSU (via connector) to the power lines on the main PCB.

So, any thoughts on the approach and any suggestions for a PSU? Google hasn’t helped me yet..., or at least, I haven’t turned up anything which delivers all four voltages (-5v being the problem).

I haven't seen a laptop with multiple voltages from the PSU for donkey's years, they all have had a single DC supply presumably with the 3.3V/5V/12V DC stepped down internally from the 15-20VDC typically supplied by the line rat.

I meant laptop format (neat box on a mains leads) rather than an actual laptop supply, which I agree normally just supply a single voltage. So something like the example herehttps://www.matrixsynth.com/2013/03/che ... -with.html- although this looks home-made and might only be bypassing the transformers, not the regulators.

Desktop supply in a box might be OK, but again, don’t think I have seen one with -5v?

Sam, yes, well aware and I have the MAAD Rom upgrade fitted, so it is worth making an effort for the old MS6. I can solder, so a kit would be fine. The biggest issue as a DIY-er has always been the PCBs. I have made them in the past but they took a long while to draw, etch, clean and drill. As I now tend to value my time more highly than in the past, going to Orchid or similar may be a good option.

Buying something off the shelf would be even better, so any other suggestions welcome. It would appear to be a non-standard set of voltages, though.

Sorry, should have said, for anyone not familiar with MS6 internals: everything, including PSU, and excepting front panel switches and display, is on a single circuit board. As the board nearly fills the inside of the rack, left to right and front to back, that’s what makes it a bit tricky to work on.

Two transformers, one for each +/-5/12v supply. I understand, from comments on the web, that the transformers are hard to come by. Given the mods around the regulators, including a flying resistor that doesn’t connect to anything else, I would prefer to bypass the lot.

OK, I'd thought this was an external PSU as well.Moving two of the regulators onto a strip of ally was probably done to keep them cooler (they can get hot enough to be uncomfortable).

The floating resistor is bad. I'd be inclined to make a note of where it was, then remove it.

I'm guessing that lump in the top corner with the pretty coloured wires is a voltage selector. If you've been blowing fuses, it's possible the contacts on that have been damaged. Some of these can be pulled right out, then repositioned for the alternative voltage range, but many are captive and you can't see the pins - I take it you have checked it's on the right setting.

I'm also assuming you've got a simple test meter.

The big 4700u cap is on the +5V supply, so (as you can easily get to both ends) you can quickly check to see if anything is happening at all. Depending on exactly what the transformer windings are, I would expect about 8-9V across it (it is wired between the transformer and the regulator so is a convenient test point.

If that's OK, I would want to get to the other three biggish caps, but that might not be so easy.

Correct on the voltage selector at top left and yes, it is set to 240, not 120 volts. Yes, I have a multimeter.

Voltage across 4700u cap is 7V.

I can take out the main PCB to get access to all components but it will be a little tricky to make all the connections safely and hold the board securely whilst applying power. Do you think a repair is a better option than a PSU replacement?

BillB wrote:Correct on the voltage selector at top left and yes, it is set to 240, not 120 volts. Yes, I have a multimeter.

Voltage across 4700u cap is 7V.

I can take out the main PCB to get access to all components but it will be a little tricky to make all the connections safely and hold the board securely whilst applying power. Do you think a repair is a better option than a PSU replacement?

Everyone has their preferences, but I generally prefer in-situ repairs where possible.

That voltage is lower than I'd like, it doesn't leave much headroom for the regulator. Still... One of the smaller caps should have a similar voltage across it - maybe slightly higher, and the other two should have about 15V.

If running with the board partly out use copious pieces of cardboard to make sure everything is insulated from the case. Cardboard is best as if it gets in contact with something very hot it just smolders. Paper might catch fire, and plastic will melt, with bits quickly poking through.

I know it is too late but fuses are generally specified at a certain value for a reason - NEVER replace one a higher rated fuse. I'd go along with Folderol's suggestion to fix what you have rather than rebuild. It sounds like a basic regulated supply which won't be hard to fix if you take a logical approach.